Recently, Internet Protocol (IP) transport solutions have been considered for 3rd generation (3G) mobile communications networks because of the flexibility and wide deployment of IP technologies. For example, the 3GPP release 8 intends to support user plane over the A interface between the radio access network and the core network using IP (AoIP) protocol [3GPP TS 48.008] and AoIP userplane transport mechanism [3GPP TS 48,103]. AoIP permits the situation where transcoder equipment may only be located in the core network (e.g., a mobile gateway (MGw)), in which case, transcoder resources are not available in the radio access network. An advantage of AoIP is the possibility and high probability to conduct a speech call in a Transcoder Free Operation mode (TrFO). AoIP together with an adaptive voice coding, e.g., Adaptive Multi-Rate Narrowband (AMR-NB), and with the ability to operate over a wide range of voice codec bit rates can use a GSM radio network with full rate and half rate traffic channels to its full potential.
Adaptive voice coding; e.g., Adaptive Multi-Rate (AMR), can be used to vary the bit rate of voice codecs for different reasons, e.g. to adapt to radio quality, to adapt to the load situation in the network, and to adapt to the width of local and remote radio interfaces. The protocol enhancements for AoIP mentioned above ensure that end-to-end codec negotiation takes place at the initial call set-up between a local mobile radio node and a remote radio node, and that a TrFO mode is maintained even when a handover is required because a mobile radio node moves or when Radio Resource Management (RRM) in the radio network must adapt codec rates in mobile radio nodes to accommodate current radio conditions and/or a current radio resource situation. If the RRM output requires a change of an ongoing TrFO call to a codec that is incompatible with that currently used, e.g., GSM_FR (full rate) to GSM_HR (half rate) or AMR-NB to GSM_EFR, then a transcoder resource must be inserted to convert between these two different codec types. In 3GPP TS 48.008, the procedure to insert such transcoder resource is called Internal BSS (Base Station Subsystem) Handover with MSC (Mobile Switching Center) support. But if the call operates using AMR codecs and the output of RRM indicates a change of traffic channel bit rate (e.g., from full rate to half rate) for an AMR-NB call, then a change to a compatible codec is assumed, which means that no transcoder resource is required, and the change is handled by BSS without support from MSC. As a result, the call can be kept in a TrFO mode.
Because the codec set used for AMR-NB on a full rate traffic channel and the codec set used for AMR-NB on a half rate traffic channel are compatible (from the decoder point of view), and thus no transcoder equipment is required when a rate/mode change occurs, there is no need for explicit control signaling to the core network or to the remote mobile radio node when a change from one to another occurs. With both ends configured with compatible codec sets, the lower codec modes for both are the same. However, the source rate of the payload generated by an AMR in the FR case with good radio conditions will not “fit” into a radio channel on the radio interface configured for AMR half rate channel. In other words, AMR codec modes with bitrates above 7.40 kbps corresponding to AMR on a full rate traffic channel with good radio conditions do not fit on a half rate radio channel which only accommodates 7.40 kbps or less.
A problem thus occurs when the traffic channel for the local mobile radio node, for example, is changed from full rate (FR) to half rate (HR) during a call with the remote mobile radio node. The local mobile radio node starts operating at a low codec mode, known as initial codec mode, and the remote mobile radio node only adapts to the new rate after it has received the information in an AMR codec mode request (CMR) message embedded in a the AMR payload from the local mobile radio node. The CMR message is the mechanism where the receiver node tells the sender node which codec modes that are the highest possible for the last step to the receiver. Consequently, the adaptation takes at least one round-trip-delay between the local mobile radio node and the remote mobile radio node, i.e., approximately 300-400 ins, starting from when the local mobile radio node has set-up the new radio channel which in the example above is when the local mobile radio node has changed from a traffic channel configured for full rate to a traffic channel configured for half rate. All the AMR frames from the remote mobile radio node to be transmitted to the local mobile radio node over the radio interface during this time will be discarded at the radio interface until the remote mobile radio node adapts its codec rate from a codec rate above what can be transmitted on a half rate traffic channel down to a codec rate that “fits” the half rate channel. As a result, the user at the local node detects audible distortion or dropouts and experiences overall decreased speech quality.
The same problem appears for an inter-BSS Handover. For example, the new local mobile radio node and the new base station may start with a low codec mode on a half rate radio channel after the inter-BSS Handover, while the remote mobile radio node might still be using a high codec mode on a full rate radio channel until the remote node is informed of the rate change.